AMHS's have been used extensively in the semiconductor fabrication field. The typical system includes a plurality of bays (rows) of storage areas. Each bay has a stocker, which includes bins for holding a plurality of containers, such as standard mechanical interface (SMIF) containers for loading 200 mm (8 inch) wafers, or front opening unified pods (FOUPs), which may be used to load 300 mm (12 inch) wafers. The stocker holds the SMIFs or FOUPs in preparation for transporting a SMIF or FOUP to the loadport of a processing tool. An overhead hoist transport (OHT) associated with each bay transports the SMIF or FOUP with wafers from the stocker to a loadport for processing in one of the tools (fabrication process machines).
Because the availability of wafers to process at the time the equipment is ready to perform the processing has a major impact on the overall production rate, it is important to operate the AMHS in a manner that supplies wafers quickly as soon as they are needed. A frequently used measure of the AMHS performance is the Operator Service Time (OST). The OST is an efficiency index of the AMHS that measures the period of time between issuance of a retrieval command for a lot of wafers (by the load port of the processing tool) and the time when the wafers are available to the operator at the tool. Two significant components of the OST include: (1) the stocker output port time, which is the period between issuance of a retrieval command by the load port of the processing tool and the time when the wafers are transferred to the stocker output port, and (2) the tool load port time, which is the period between issuance of a retrieval command by the load port of the processing tool and the time when the wafers are transferred to the load port of the equipment.
OST is heavily influenced by the way in which the stockers are controlled. One aspect of stocker control is the way in which the utilization of each stocker was managed. If one stocker has all of its bins full, while another stocker is empty, then the stockers cannot efficiently supply wafers when they are needed, and processing will be delayed. High stocker utilization leads to low stocker hit ratio (i.e., the wafers are less likely to be stored in the stocker nearest the tool where they are needed). This results in greater OST, reducing the tool efficiency (because the tool cannot be used 100% of the time when it is not constantly supplied with wafers).
Typically, when the utilization of the stocker exceeded a maximum utilization specification, an alarm system sent a message to an on-duty operator. The on-duty operator would trigger a command to move one or more appropriate FOUPs using an materials control system (MCS) server graphical user interface (GUI), until the utilization again was within the specification. One or more FOUPs would be moved from the stocker with excess utilization to a stocker with lower utilization. The on-duty operator required time to decide which FOUPs to move, and to which destination stocker the FOUPs should be relocated. Thus, balancing the stocker utilization manually required time and manpower.
It would be desirable to provide a method and system that improves the stocker output port time, and thus improves the OST.